Thermoplastic adhesives, otherwise known as “hot melt” adhesives have been widely used in industry for adhering many types of products. Hot melt adhesive dispensing systems generally include one or more dispensing guns, heated hoses connected to the guns, and a dispensing unit for melting and supplying heated liquid adhesive to the guns through the heated hoses. The dispensing units of conventional hot melt adhesive systems include a tank, a heater, a pump, a manifold, and a controller. The heater is located in a base of the tank and the tank is mounted on top of the manifold so that the heater can heat both the tank and the manifold. The manifold has an inlet connected to the tank and typically has multiple outlet ports for connection to the heated hoses. Adhesive material is supplied to the tank in solid or semi-solid form, where it is melted and heated to a desired application temperature. The pump associated with the tank and manifold pumps liquid adhesive from the tank, through the manifold and heated hoses to the dispensing guns. The controller is generally located adjacent the tank and controls the power supplied to the heater and heated hoses to maintain the liquid adhesive at an appropriate viscosity and temperature, depending on the application. The controller typically controls many other system operations as well.
While conventional dispensing units for hot melt adhesive systems, as described above, have been in use for many years, improvements in design are still needed. For example, the location of the manifold beneath the tank creates an asymmetric layout wherein the heated hoses connecting the outlet ports of the manifold to the adhesive guns must be of different length depending upon whether the gun is located adjacent the tank or on an opposite side of the dispensing unit adjacent the controller. This asymmetry can be problematic when a single dispensing unit is positioned between two manufacturing lines to supply adhesive to the two lines. It can also be problematic if separate dispensing units are used to supply separate manufacturing lines. In each case, as a consequence of the asymmetry of current dispensing units, end users must purchase and stock heated hoses of various lengths to accommodate various arrangements of dispensing systems.
Also, utilizing a common heater to heat both the tank and the manifold can expose the adhesive flowing through the manifold to more heat than is often necessary for maintaining proper adhesive viscosity and temperature. This is especially problematic when the heater is energized to melt solid or semi-solid adhesive that has been added to the tank during operation when the tank level is low. This problem is sometimes referred to as thermal shock and is caused by the control system's attempt to rapidly melt the relatively cool, solid or semi-solid adhesive added by the operator. The rapid, increased heating can cause undesirable charring and degradation of adhesive in the manifold and thereby cause various adhesive performance problems.
Another challenge with conventional hot melt adhesive systems relates to the time period needed to heat the pump to operating temperature at start-up. Since the pump is located within the tank, it is only indirectly heated by the hot adhesive in the tank. This typically lengthens the start-up time. Location of the pump in the tank also minimizes the adhesive holding capacity of the tank and limits the size of the effective tank opening for receiving new adhesive into the tank.
For at least these reasons, a need exists for a dispensing unit of a hot melt adhesive system having a configuration allowing the user to stock heated hoses of more uniform length. A need also exists for a dispensing unit for a hot melt adhesive system which facilitates more precise temperature control of adhesive flowing through the manifold and which heats the pump faster at start-up.